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The Collisional Ejection Theory

Currently, the theory that best explains all the evidence is the
collisional ejection theory. This theory holds that the Moon formed
from debris ejected from the Earth when a large object (possibly as
large as Mars) crashed into the Earth. Simulations of this scenario
show that energy from such a collision produces a stream of vaporized
rock from the impact. The Moon forms from this cooled material.

This theory explains many of the known properties of the Moon's orbit
and composition. The ejected material would have coalesced in or near
the ecliptic plane, putting the Moon into an orbit much like the one
it has. The Moon, which is believed to have a small iron core, could
have retained this core through the collision. Volatile elements
would have been vaporized during the impact. Also, this collision
could have tipped the Earth's axis, causing the seasons we now know.

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